Automatic transmissions for automotive vehicles of contemporary design typically include multiple-ratio gearing having torque input gear elements connected drivably to an internal combustion engine through a hydrokinetic torque converter. A driven element of the gearing is drivably connected to a differential axle assembly for vehicle traction wheels. Relative motion of the elements of the gearing is controlled by clutches and brakes that establish plural torque flow paths through the gearing, each torque flow path being associated with a discrete torque ratio.
Ratio changes during forward-drive operation of the vehicle are achieved by engaging and disengaging a friction element, which may be a friction clutch or a friction brake, in the torque flow path. Further, overrunning couplings may cooperate with the friction element during a speed ratio change. As a friction element is engaged, an overrunning coupling is released. When the friction element is disengaged, the overrunning coupling element establishes a mechanical torque flow path through associated gearing elements.
An example of a known multiple-ratio automotive transmission mechanism in an automotive vehicle powertrain may be seen by referring to U.S. Pat. Nos. 5,150,297, 5,081,886, 4,509,389, 5,303,616, 5,474,506 and 5,383,825, each of which is assigned to the assignee of the present invention.
The lowest speed ratio in a transmission of the kind disclosed in the foregoing patents includes an overrunning coupling that serves as a torque reaction brake during operation in the lowest speed ratio. A nonsynchronous upshift to the second lowest speed ratio is achieved by engaging a friction clutch, thereby establishing a nonsynchronous 1-2 upshift. Nonsynchronous ratio changes from the second ratio to the third ratio and from the third ratio to the fourth ratio also are achieved by selectively engaging friction clutches as the state of a companion overrunning coupling changes from a torque delivery mode to an overrunning mode. Such shifts commonly are referred to as nonsynchronous shifts since only a single friction element is involved in the ratio change. The other coupling, which may either be an overrunning brake or an overrunning clutch, need not be disengaged nor released in synchronism with the application or a release of the friction element because the overrunning coupling freewheels the instant the torque flow path through the overrunning coupling is interrupted during a ratio change.
Another example of a known transmission system capable of being used in an automotive vehicle driveline includes a compound planetary gear unit in combination with a simple planetary gear unit, the compound planetary gear unit having three forward-driving ratios. When the simple planetary gear unit is combined with the compound planetary gear unit, either one or two additional ratios are obtained. An example of a transmission of this type can be seen by referring to U.S. Pat. Nos. 5,758,302, 5,586,029 and 5,722,519. These patents, as in the case of the previously described prior art patents, are owned by the assignee of the present invention.
The gearing systems of the '302, '029 and '519 patents have a friction brake and a friction clutch for controlling the relative motion of the elements of the simple planetary gear unit. An overrunning coupling establishes and disestablishes a driving connection between the elements of the simple planetary gear unit. A nonsynchronous 1-2 upshift is achieved as the friction brake is applied and the companion overrunning coupling freewheels. As in the case of the transmission previously described, this 1-2 upshift is a nonsynchronous upshift.
The control systems for such contemporary transmissions are designed initially for the best possible shift quality. They are not capable, however, of accommodating unit variations in the clutch and brake actuators. Neither are they capable of accommodating system characteristic changes during the life of the transmission as the friction elements are subjected to wear.